Bandpass filters are used in many applications including radio frequency receiver paths and bandpass delta-sigma analog-to-digital converters. Typical bandpass filter implementations require inductors, and it is difficult to implement high quality, appropriately sized, inductors in complementary metal oxide semiconductor (CMOS) processes. Other options exist for implementing bandpass filters, but they tend to have issues with performance and/or power, or require alternative process technologies.
N-path filters are a practical method for implementing high-Q bandpass filters in modern CMOS processes without inductors using a combination of mixers and low-pass filters. The Q factor is the ratio of the center frequency of the filter to the pass band bandwidth. The basic structure of an N-path filter is multiple paths, each path composed of a mixer, filter and mixer, summed together to form the filter output. With trends in process scaling leading to higher switching frequencies, N-path filters are a viable option for integrated bandpass filter designs with center frequencies of interest in current communication standards. As the center frequency of the filter is decoupled from the bandwidth of the filter, high Q values are achievable. The mixer sequences have a high impact on the performance of the N-path filter. Therefore optimizing practically realizable mixer sequences allows optimization of the N-path filter.